Vacuum type circuit interrupter



April 18, 1961 H. N. SCHNEIDER VACUUM TYPE CIRCUIT INTERRUPTER FiledMarch 26, 1959 Inventor: Harold N. Schneiden His Attorneg.

breakdown along the insulating surface.

United States Patent O VACUUM TYPE crncurr INTERRUPTER Harold N.Schneider, Springfield, Pa.. assignor to General Electric Company, acorporation of New York Filed Mar. 26, 1959, Ser. No. 802,235

9 Claims. (Cl. 200-144) This invention relates to vacuum-typecircuitinterrupter and, more particularly, to an improved shielding arrangementfor protecting the insulation of the interrupter against the build-up ofmetallic coatings thereon.

In a vacuum circuit interrupter, the metallic vapors that are producedby arcing tend to condense on the insulating surfaces of the interrupterand, hence, to form metallic coatings which impair the insulatingproperties of such surfaces. For protecting these surfaces against suchmetal deposition, a shielding arrangement of the general type disclosedand claimed in application Serial No. 630,- 247, Crouch, now Patent No.2,892,911, filed December 24, 1956, and assigned to the assignee of thepresent invention, has been found effective. Such shielding arrangementcomprises a generally-tubular metallic shield surrounding the arcing gapbetween the usual electrodes of the interrupter and electricallyisolated from both electrodes and from ground. This electricallyisolated shield is physically located between the arcing gap and theinternal surface of the usual insulating casing for the interrupter andacts to intercept and condense those metallic vapors traveling towardthe insulating surfaces from the arcing gap.

The shield can be constructed so that very few of the particlesliberated by each interruption reach the internal surface of the casing.But, in some cases, after an extended number of operations, the numberof particles reaching the internal surface will become significant. Inthis regard, deposition of a significant number of these particlesresults in the build-up of a metallic coating along the internalinsulating surface of the interrupter casing. Such coating would reducethe creepage distances present in the interrupter, thus increasing thepossibility of a One possible approach toward reducing the possibilityof such a breakdown would be to increase the length of the insulatingcasing so the longer creepage paths would be available to withstand theapplied voltage. But this approach is disadvantageous because itinvolves unduly increasing the overall dimensions of the interrupter andalso because it involves exposing the added insulating surfaces of thelengthened casing to metal deposition, unless the metal shield is alsolengthened or otherwise reconstructed.

Accordingly, an object of my invention is to construct a vacuuminterrupter of this general type in such a manner as to provideincreased creepage surfaces, protected from metal deposition, yetwithout increasing the bility that this bake-out step will causemetallic vapors or impurities driven off from the outer surface of themetallic shield to be condensed or deposited on the insu- P latingcasing of the interrupter. In prior arrangements, such deposition couldundesirably decrease the creepage distances available in theinterrupter.

Thus, another object of my invention is to mount the metallic shield insuch a manner that any metallic vapors or impurities driven off from theouter surface of the shield during bake-out, or during other elevatedtemperature conditions, do not materially impair the creepage distancespresent in the interrupter.

In carrying out my invention in one form, I provide a highly evacuatedenvelope comprising a generally tubular casing of insulating material.Disposed within the envelope is a pair of electrodes having aspaced-apart position defining an arcing gap therebetween. A generallytubular metallic shield surrounds the arcing gap and extends forsubstantial distances on opposite sides of the arcing gap so that itsinternal surface can intercept and condense metallic vapors travellingfrom the arcing gap toward the internal insulating surface of thecasing. The meallic shield is electrically isolated from both electrodesand from ground and is supported on the casing by means of a generallytubular shield-support of insulating material. This tubularshield-support surrounds the shield and extends along the length thereofbetween spacedapart points located near the ends of the shield. Theshield is attached to the tubular shield-support, preferably at thesespaced-apart points, and the tubular shieldsupport is attached to thecasing at a single longitudinallyrestricted location. Aside from at thislongitudinallyrestricted location, the tubular shield-support is spacedfrom thecasing at all points along the length of the shield-support.

For a better understanding of my invention, reference -may be had to thefollowing description taken in conjunction with the accompanyingdrawing, wherein:

Fig. 1 is a cross-sectional view of a vacuum-type circuit interrupterembodying my invention in one form.

Fig. 2 illustrates a modification of one feature of the interruptershown in Fig. 1.

Fig. 2a illustrates another modification of one feature of theinterrupter shown in Fig. 1.

Fig. 3 illustrates still another modified form of the general type ofvacuum circuit interrupter shown in Fig. 1.

Referring now to the interrupter of Fig. 1, there is shown ahighly-evacuated envelope 10 comprising a tubular casing 11 ofinsulating material, such as a suitable glass, and a pair of metallicend caps 12 and 13 closing off the ends of the casing. Suitable seals 14are provided between the end caps and the casing to render the envelope10 vacuum-tight.

Located within the envelope are a pair of separable electrodes, or rodcontacts, 17 and 18 shown by solid lines in the closed-circuit position.The electrode 17 is a stationary electrode suitably united to the upperend cap 12, whereas the electrode 18 is a movable electrode suitablymounted for vertical movement and projecting through an opening in thelower end cap 13. A flexible metallic bellows 20 interposed between theend cap 13 and the movable electrode 18 provides a seal about themovable electrode and allows for vertical movement thereof withoutimpairing the vacuum inside the interrupter. As shown on the drawing,the bellows 20 is sealingly secured at its respective opposite ends tothe electrode 18 and the end cap 13.

Coupled to the lower end of the movable electrode 18, I provide suitableactuating means (not shown) which is capable of driving the electroderapidly downwardly from its solid-line position of Fig. l to itsdotted-line position to open the interrupter and which is also capableof returning the electrode to the solid-line position to closetheinterrupter.

When the electrode is driven downwardly to open the interrupter, acircuit-interrupting, or arcing, gap is estab-* lished between theadjacent ends of the electrodes, and the resulting arc, though quicklyextinguished,.vaporizes some of the metal of the electrodes.

In order to prevent this metallic vapor from condensing on the internalinsulating surfaces of the casing 11, there is provided a metallicshield 25 which corresponds in certain respects to a similarlydesignated shield in the aforesaid Crouch application. This metallicshield 25 is of a generally tubular configuration and extends along thelength of the casing 11 for substantial distances on opposite sides ofthe gap between the electrodes. The shield 25 is electrically isolatedfrom both of the electrodes 17 and 18 and, preferably, is also isolatedfrom ground, or in other words, is at a floating potential relative tothe two electrodes.

For achieving this electrical isolation between the shield 25 and theelectrodes, I have supported the shield 25 on the casing 11 by means ofa shield-support 27 formed of an insulating material, preferably glass.This shield-support 27, which is preferably of a generally tubularconfiguration, surrounds the shield 25 and extends along the length ofthe shield 25 between spaced-apart points located near the opposite endsof the shield 25. At these spaced-apart points, suitable fastening meanssuch as 28 are provided for attaching the shield 25 to the tubularshield-support. In Fig. 1 these fastening means comprise U-shapedmetallic finger welded to the outer surface of the metallic shield 25and crimped around the ends of the tubular shield-support 27.

The tubular shield-support 27 is supported on the casing 11 byattachment means in the form of a joint 30 which constitutes the solemechanical connection between the shield-support 27 and the casing 11.This joint 39 extends along a single longitudinally-restricted portionof the shield-support '27 substantially shorter in length than thelength of the tubular shield-support 27. As a result, the tubularshield-support 27 is spaced from the casing 11 at all points along itslength except at the joint 30. Preferably, the joint 30 is locatedcentrally of the length of the tubular shield-support 27.

The parts 27 and 11 can be formed from any suitable combination ofstructural components fused together at the joint 30. One desirable wayof forming these parts is to form the casing 11 and the shield-support27 each as separate tubular components, which are then telescoped andjoined together about the outer periphery of the shield-support by asuitable fusion process which integrally unites them at the joint 30.Another desirable method of construction is to provide two alignedtubular parts, one corresponding to the top half of the shieldsupportand the casing and the other to the bottom half, and to fuse these twoparts together along a horizontallyextending seam at the joint 30.

It Will be apparent from Fig. 1 that essentially all straight line pathsextending from the general region of the arcing gap to the insulatingcasing 11 are intercepted by the floating central shield 25. As a resultof this relationship, substantially all metallic particles that areliberated from the arcing tips by arcing are captured and condensedeither by the shield 25 or by the end caps 12 and 13. A small percentageof these metallic particles can bounce off the metallic parts one orpossibly several times before finally adhering, but most of theseparticles will be captured on the metallic parts before they can reachthe internal surface of the insulating casing 11.

Although very few of the metallic particles liberated by eachinterruption reach the internal surface 31, 32 of the casing 11, thenumber of particles reaching the internal surface might, in some cases,become significant after an extended number of interrupting operations.In this regard, it will be apparent that such particles would tend tocoat this internal surface and thus reduce the length of the creepagepaths present in the interrupter.

The term creepage path, as used herein, denotes the path along aninsulating surface over which an electrical breakdown may occur betweentwo conductive members between which voltage exists. The shorter thesecreepage paths become, the greater becomes the likelihood of anelectrical breakdown along the surface.

Because of the particular manner in which my shield 25 is mounted, theinterrupter of the present invention has exceptionally long creepagepaths in comparison to prior interrupters of equal casing length. Moreparticulanly, the creepage path in the disclosed interrupter between thetwo end caps 12 and 13 does not extend merely along the internalinsulating surface 31, 32 of the casing 11 as has been the case in priorinterrupters, but in addition extends along the outer surface 33, 34 ofthe tubular shield support 27 Thus, the minimum creepage path betweenthe two end caps 12 and 13 in the disclosed interrupter is constitutedby the sum of the lengths of surfaces 31, 33, 34, and 32. It willtherefore be apparent that if any portion of the casing insulatingsurface does become metallically coated, this coating will be much lesslikely to cause an electrical breakdown than would be the case with aninterrupter having shorter creepage paths. The fact that the outersurface 33, 34 of the tubular shieldsupport 27 are protected from metaldeposition helps to maximize the ability of these surfaces to provideadequate creepage distances after an extended number of switchoperations.

The metallic shield 25 of the disclosed interrupter is preferably at amid-potential relative to the two electrodes l7 and 18 when theseelectrodes are in their fully open position. This follows from the factthat the gaps separating each end of the shield 25, 28 and the adjacentend cap are of substantially the same size and configuration. Theselatter relationships would not be materially changed by the presence ofa metallic coating on the internal surface of the shield 25, and thusthe production of such coatings on the shield 25, as a result of arcing,does not significantly vary the mid-potential relationship of the shield25 relative to the electrodes.

In accordance with the aforementioned Crouch application, I prefer toconstruct the interrupter in such a manner that the electric field inthe region of the arcing gap is generally symmetrical with respect to areference plane which bisects the arcing gap between the fully openelectrodes and extends perpendicular thereto. Because of the symmetricalrelationship of the electrical field about the arcing gap, nosignificant polarity effect is present, i.e., the gap has substantiallythe same breakdown strength when subjected to voltage of one polarity aswhen subjected to voltage of an opposite polarity. Perhaps the mostimportant factor contributing to this symmetrical field relationship isthe fact that my shield 25 is at approximately mid-potential relative tothe two electrodes 17 and 18 when these electrodes are in theirfully-open position. The fact that most of the conductive parts of theswitch are generally symmetrical with respect to the above-describedreference plane also contributes to the symmetry of the electric field.

It will be apparent that should the position of the shield 25 change forany reason, such changes could affect the potential of the shield andalso the configuration of the electric field, causing significantdepartures from the above-described ideals. An important advantage of mydisclosed shield-support 27 is that it is especially wellsuited tomaintaining the desired position of the shield within close limits. Inthis regard, it will be noted that the shield-support 27 supports theshield 25 at widelyspaced points near its ends, rather than at a singlerestricted location. As a result, the disclosed shield-support forms anexceptionally stable mounting for the shield 25.

One of the usual steps in the manufacture of a vacuumtype interrupter isa bake-out step in which the interrupter is baked at a relatively hightemperature to drive off adsorbed gases and other impurities from thesurfaces of its parts. It is, of course, highly desirable thatthesevapors and impurities be prevented from coating the crucial insulatingsurfaces of the interrupter. The shieldsupport of the present inventionis of considerable assistance in this regard inasmuch as its innersurface 36 serves to capture any particles driven off from the outersurface of the shield 25. The presence of a conductive coating on thissurface 36 does not lessen the creepage distances present in theinterrupter inasmuch as this surface is already bridged by theconductive shield 25.

Should the shield temperature become elevated for any reason, forexample, as a result of an extreme heavy current interruption, it willbe apparent that the innersurface 36 of the shield-support 27 will actin the same manner as described hereinabove to capture particles emittedfrom the outer surface of the shield 25.

It is to be understood that the fastening means 28 shown in Fig. 1 forconnecting the shield 25 to the shieldsupport 27 is merely illustrativeof one embodiment of my invention. For an alternative form of fasteningmeans, reference may be had to Fig. 2, where an annular adaptor 40,S-shaped in cross-section, is used for connecting the shield-support 25to the shield-support 27. This S-shaped adaptor 40 is welded at one endto the shield 25 and is joined at its other end to the shieldsupport 27by being imbedded in the shield-support. The adaptor 40 has a moderatedegree of yieldability and, accordingly, allows the shield 25 to expandgenerally independently of the shield-support 27 during changingtemperature conditions. Thus, during bake-out or other high temperatureconditions, expansion of the shield 25 does not objectionably stress theshield support 27. For another alternative form of fastening means,reference may be had to Fig. 2a. Here an adaptor 40a generallycorresponding to the adaptor 40 of Fig. 2 is joined at its outer end tothe shield-support27 by being crimped or otherwise fitted around theouter end of the shieldsupport 27. This adaptor also has a moderatedegree of yieldability for the purpose of allowing the shield 25 toexpand and contract generally independently of the shield-support 27during changing temperature conditions.

Although I prefer to form the casing 11 as a tube of substantiallyuniform diameter and the shield-support 27 as a centrally expanded tube,as shown in Fig. 1, it is to be understood that these shapes can bematerially varied without departing from the broader aspects of myinvention. For example, as illustrated in Fig. 3, I prefer to form thecasing 11 of certain interrupters with a contracted central region andthe shield-support 27 as a tube of substantially uniform diameter. Thecasing 11 and the shield-support 27 are once again integrally united asby a suitable fusion process.

Another advantage of both of these constructions is that noglass-to-metal seal is required for supporting the shield 25.Eliminating such a seal helps to reduce the cost of the interrupter andalso eliminates a possible source of leakage.

The particular interrupters shown and described are merely embodimentsof my invention, and it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from myinvention in its broader aspects. I therefore, intend in the appendedclaims to cover all such changes and modifications as wall within thetrue spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a vacuum circuit interrupter, a highly evacuated vacuum-tightenvelope comprising a generally tubular casing of insulating material, apair of electrodes located within said insulated casing and disposed inspaced-apart relationship during a circuit-interrupting operation todefine an arcing gap therebetween, a generally tubular metallic shieldsurrounding said arcing gap and extending along the length of saidcasing for 6 1 substantial distances on'opposite sides of said arcinggap for intercepting and condensing metallic vapors emitted from saidarcing gap, said metallic shield having a potential relative to one ofsaid electrodes in open-circuit position which is a predeterminedpercentage of the potential between electrodes, said percentage beingretained substantially unchanged by the condensation of said metallicvapors on said shield even from a time prior to said condensation, saidshield being electrically isolated from both of said electrodes and fromground, a tubular shieldsupport of insulating material surrounding saidshield and extending along a major portion of the length of said shield,means for attaching said shield to said tubular shield-support, meansfor supporting said shield-support on said casing in spaced-apartrelationship relative to said casing in such a manner that the minimumlength creepage path between said shield and said electrodes extendsalong the outer surface of said tubular shieldsupport for a substantialportion of the length of said shield-support, said interrupter beingsubstantially devoid of solid insulating material located between saidarcing gap and said metallic shield.

2. In a vacuum circuit interrupter, a highly-evacuated vacuum-tightenvelope comprising a generally tubular casing of insulating material, apair of electrodes located within said insulated casing and disposed inspaced-apart relationship during a circuit-interrupting operation to de:fine an arcing gap therebetween, a generally tubular metallic shieldsurrounding said arcing gap and extend-- ing for substantial distanceson longitudinally opposite sides of said arcing gap for intercepting andcondensing metallic vapors emitted from said arcing gap, said metallicshield having a potential relative to one of said electrodes inspaced-apart position which is a predetermined percentage of the voltagebetween said electrodes, said percentage being retained substantiallyunchanged by the condensation of metallic vapors on said shield evenfrom a time prior to the condensation of metallic particles thereon,said shield being electrically isolated from both of said electrodes andfrom ground, a shield-support of insulating material disposed about theouter surface of said shield and extending along a major portion of thelength of said tubular shield, means for fastening said shield to saidshield-support, attaching means for attaching said shield-support tosaid casing and constituting the sole mechanical connection between saidshield-support and said casing, said attaching means extending along alongitudinally-restricted portion of said shield-support substantiallyshorter in length than the length of said shield-support and spaced fromthe ends of said shieldsupport, said shield being interposed betweensaid arcing gap and said attaching means for protecting said attachingmeans from the condensation of arc-liberated metallic particles thereon,said interrupter being substantially devoid of solid insulating materiallocated between said arcing gap and said metallic shield.

3. The vacuum circuit interrupter of claim 2 in which saidshield-support is of a generally tubular configuration and extends alongthe length of said shield between spaced-apart points located near theends of said shield, and in which said means for fastening the shield tosaid shield-support comprises fastening means located at saidspaced-apart points.

4. The vacuum circuit interrupter of claim 2 in which said shieldsupport is of a generally tubular configuration and surrounds saidshield.

5. In a vacuum circuit interrupter, a highly-evacuated vacuum-tightenvelope comprising a generally tubular casing of insulating material, apair of electrodes located within said insulating casing and disposed inspaced-apart relationship during a circuit-interrupting operation todefine an arcing gap therebetween, a generally tubular metallic shieldsurrounding said arcing gap and extending for substantial distances onlongitudinally-opposite sides of said arcing gap for intercepting andcondensing metallic vapors emitted from said material disposed aboutsaid shield and extending along a major portion of the length of saidshield, means for attaching said shield to said shield-support, meansconfined to a longitudinally-restricted location spaced from the ends ofsaid shield-support for supporting said shieldsupport on said casing inspaced-apart relationship relative to said casing at all points alongthe length of said shield-support except at saidlongitudinally-restricted location.

6. The vacuum interrupter of claim in which said shield-support is of agenerally tubular configuration and surrounds said shield. v

7. The vacuum interrupter of claim 5 in which said shield-support is ofa generally-tubular configuration and extends along the length of saidshield between spacedapart points located near the ends of said shield,and in which said means for attaching said shield to said shieldsupportcomprises fastening means located at said spacedapart points.

8. The interrupter of claim 5 in which said means for attaching saidshield to said shield-support is yieldable to allow forthermally-induced expansion of said metallic shield relative to saidshield-support.

9. In a vacuum circuit interrupter, a highly-evacuated vacuum-tightenvelope comprising a generally tubular casing of insulating material, apair of electrodes located within said insulating casing and disposed inspaced-apart relationship during a circuit-interrupting operation todefine an arcing gap there-between, a generally tubular metallic shieldsurrounding said arcing gap and extending for substantial distances onlongitudinally opposite sides of said arcing gap for intercepting andcondensing metallic vapors emitted from said arcing gap, saidinterrupter having insulating surfaces which when coated with conductivematerial will form a conductive path which lowers the creepage distancebetween said electrodes and between said shield and said electrodes,said shield being so located that it is interposed between substantiallyall of said insulating surfaces and said arcing gap, a tubularshield-support of insulating material surrounding said shield andextending along the length of said shield between spaced-apart pointslocated near the ends of said shield, means for attaching said shield tosaid tubular shield-support at said spacedapart points, means locatedgenerally centrally of the length of said tubular shield-support forsupporting said shield-support on said casing in spaced-apartrelationship to said casing at all points along the length of saidshieldsupport except generally centrally of the shield-support length.

References Cited in the file of this patent UNITED STATES PATENTS1,011,160 Coflin Dec. 12, 1911 2,863,026 Jennings Dec. 2, 1958 2,864,998Lee Dec. 16, 1958 2,892,911 Crouch June 30, 1959 2,897,322 Reece July28, 1959 FOREIGN PATENTS 379,342 Great Britain Aug. 29, 1932 389,463Great Britain Mar. 6, 1933 787,846 Great Britain Dec. 18, 1957

